Recently, liquid crystal displays have come into the limelight as one of the most competitive display devices in place of cathode ray tubes in view of their various merits such as light weight and low power consumption. In particular, since a thin film transistor liquid crystal display (TFT-LCD) driven by thin film transistors (TFTs) can independently drive respective pixels, it ensures a rapid response time of liquid crystals, which enables high-definition video pictures. Accordingly, the TFT-LCD has increasingly expanded to its application fields of notebook computers, wall-mounted televisions, etc.
Generally, since linear polarizers are disposed outside both glass substrates in the fabrication of color TFT-LCDs, the color TFT-LCDs are used to display information by adjusting the amount of light transmitted from the rear side through the actions of liquid crystal molecules between the glass substrates.
Each of the linear polarizers disposed on the glass substrates generally includes a polarization film, which comprises a base, such as a film made of oriented polyvinyl alcohol or derivatives thereof, and polarization materials including an iodine or dichroic dye and applied to the substrates.
An iodic polarization film has excellent initial polarization performance in the range of visible rays. However, the iodic polarization film not only has considerably lagging polarization performance in the range of near-ultraviolet rays, but is also vulnerable to water and heat. Further, when it is used in a high temperature humidity condition for long time, its durability becomes deteriorated.
To improve the durability, it is considered to treat the polarization film with an aqueous solution which contains formalin or a boric acid or to use a polymer film with low moisture permeability as a protective film for the polarization film. However, these methods fail to sufficiently improve the durability.
Meanwhile, a dye-based polarization film comprising dichroic dyes as a polarizing material generally has superior durability at high temperature and humidity as compared to the iodic polarization film. Thus, it is used for automobile dashboards, planes, liquid crystal projectors, etc. which require excellent heat resistance and durability. However, the dye-based polarization film has limited heat resistance and durability depending on the kind of dye and polymer.
KR Patent Laid-open Publication No. 2006-0063753 discloses a polymerizable mesogenic formulation which comprises a polymerizable mesogenic compound having a polymerizable group, a dichroic dye, a chain transfer agent, a polymerization initiator, and a solvent, a polymer film using the same, a polarizer including the film, and a use thereof in identification of safety mark.
JP Patent Laid-open Publication No. 2001-091747 discloses a method of manufacturing a liquid crystal cell substrate having excellent thin and lightweight properties, heat resistance, shock resistance and stable quality. The liquid crystal cell substrate includes a gas barrier layer, a cross-linked resin layer, and a polarizing layer, wherein the polarizing layer is formed of a dichroic dye having lyotropic liquid crystal properties, liquid crystal polymer layer containing a dichroic dye, or a lyotropic material containing a dichroic dye.
U.S. Pat. No. 6,337,111 discloses a thin film having light absorption anisotropy, which makes it easy to perform optical functions, such as attenuating, polarizing, scattering or shielding light including laser light and natural light and a process for preparing the same. According to the US patent document, an alignment film is formed on a glass and plastic substrate, and a dichroic dye solution is applied to the alignment film. Then, a solvent is removed from the solution, thereby forming an anisotropic thin film showing light absorption characteristics. The dichroic dye is azo or anthraquinone groups and has liquid crystal properties and photo- or heat-crosslinking reactivity.
JP Patent Laid-open Publication No. 2003-232919 discloses a polarizing plate, which has good properties in view of scratch resistance, processability, yield and productivity, a method for manufacturing the same, and an antireflection layer or an optical element using the same. Further, a composition comprising a photoreactive dichroic dye containing an alignment compound and a photoreactive compound is used to form a polarizing film.
The aforementioned processes have limits in mixing dichroic dyes and polymerizable liquid crystals, and thus the polarization films also have limited polarizing degrees. In the case of the polarization film of lyotropic liquid crystals, the polarization film has a region minutely aligned by a shear force, which decreases an overall polarizing degree. In addition, conventional techniques are still not sufficient to form a polarization film which satisfies superior polarizing performance and durability at high temperature and humidity. Accordingly, there is a need of a polarization plate that has excellent polarizing performance and durability under high temperature and humidity.